1. Field of the Invention
The present invention relates to a burner for burning liquid fuel used for a combustion device of a heating source such as a boiler, heating furnace and the like, and more particularly relates to a burner for burning liquid fuel by which liquid fuel is atomized together with an atomizing medium.
2. Related Art of the Invention
Conventionally, in the burner for burning liquid fuel, the following methods have been adopted for inhibiting the generation of nitride (referred to as NOx hereinafter): self exhaust gas recirculating system, two stage combustion method, three stage combustion method, and exhaust gas recirculating method. Also, various combustion systems have been proposed, in which water jet or steam jet is adopted or the furnace load is reduced or the combustion air temperature is lowered.
According to the aforementioned conventional combustion systems, combustion is gently conducted when the flame temperature is lowered and the oxygen concentration is reduced, and due to the effect of gentle combustion, it is expected that the generation of NOx is inhibited while the generation of a certain amount of soot and dust is allowed.
That is, in the conventional combustion systems, it is difficult to concurrently inhibit the generation of NOx and that of soot and dust.
As an example of conventional burners for burning liquid fuel, there is a burner in which the injection nozzles provided close to the burner tip are disposed at regular intervals or in an arrangement in which the injection nozzles are arranged approximately at regular intervals.
However, in this type burner, integrated flames are usually generated. Therefore, although the generation of soot and dust can be inhibited, the flame layers become thick and large, so that the radiation properties are deteriorated and the flame temperature is raised. Accordingly, the residence time of combustion gas in a high temperature region is increased. Therefore, it is impossible to inhibit the generation of NOx.
In other words, it is difficult to concurrently inhibit the generation of NOx and that of soot and dust by the conventional burner structure for burning liquid fuel.
Moreover, in order to inhibit the generation of NOx, a burner structure in which flames are divided is effective, and the smaller the division angle is, the more effect can be provided.
However, according to the aforementioned structure, it is impossible to avoid the delay of contact between the flames and air. As a result, the flame length is increased, so that a large amount of soot and dust is generated. Therefore, in the conventional burner, the generation of soot and dust is inhibited when the division angle is increased to not less than 30.degree.. In this case, a sacrifice is made of the generation of NOx for the sake of inhibiting the generation of soot and dust.
On the other hand, a burner for burning liquid fuel is well known, in which an atomizing medium such as steam and air is mixed with liquid fuel and this mixed fluid is atomized by a plurality of injection nozzles.
In the fuel atomizing system of the aforementioned burner for burning liquid fuel, particles of liquid fuel mixed with the atomizing medium are made to be minute and dispersed by the expansion energy generated when an atomizing medium such as steam and air is injected from a side of high pressure to that of low pressure.
The following two systems are well known as the aforementioned fuel atomizing system. One is an intermixing system in which the injection amount is controlled while a difference between the pressure of atomizing medium and that of liquid fuel is maintained to be constant, and the other is an intermediate-mixing system in which the injection amount is controlled by changing the pressure of liquid fuel while the pressure of atomizing medium is maintained to be constant.
When the intermixing system and the intermediate-mixing system are compared, the intermediate-mixing system is superior to the intermixing system because the consumption of atomizing medium of the intermediate-mixing system is smaller than that of the intermixing system and more minute particles of liquid fuel can be provided.
However, the following problem is caused in the atomizing systems described above. That is, liquid fuel, which is an incompressible fluid, is not provided with dispersion force, so that the atomizing medium must be maintained at high temperature and high pressure.
Further, in the case of the intermediate-mixing system in a system in which the atomizing medium and liquid fuel are mixed with each other in a reverse-Y-shaped jet flow, particles of liquid fuel can not be made to be uniformly minute and deviation is caused in the injection nozzles, depending on the pressure and flow amount of the atomizing medium and liquid fuel. For that reason, the fuel particles are not sufficiently contacted with air, and the flame length is increased, so that the combustibility is affected.
In order to solve the aforementioned problems caused in this intermediate-mixing system, a technique has been proposed, in which liquid fuel is swirled and further dispersed by the centrifugal force so that the fuel particles can be made to be minute, and while the combustion condition is improved, the fuel particles are shorn by the atomized medium (disclosed in U.S. Pat. No. 2,933,259).
It can be considered to apply the aforementioned technique in which liquid fuel is swirled and further dispersed by the centrifugal force so that the particles of the liquid fuel can be made to be more minute and the fuel particles are shorn by the atomizing medium while the combustion condition is improved, to a burner for burning liquid fuel composed of a fuel supply member and a burner tip connected with the end portion of the fuel supply member.
In this case, it is possible to cut an injection nozzle portion on the bottom surface of the burner tip so as to form a swirling section in which liquid fuel is swirled. However, according to the aforementioned structure, a portion of the injection nozzle with respect to its longitudinal direction is used for the swirling and shearing section, so that the length of the injection nozzle is substantially reduced, and the necessary length can not be ensured for the mixing portion of the injection nozzle in which fuel and atomizing medium are mixed. Moreover, even when an atomizing angle of the injection nozzle and a division angle formed by two adjoining injection nozzles are slightly changed, it is necessary to manufacture a burner tip including the swirling section for liquid fuel which must be manufactured through a high grade of machining. Therefore, the manufacturing properties can not be improved.
Moreover, with respect to the structure to supply the atomizing medium to the injection nozzle for shearing fuel particles, it can be considered to adopt a structure in which a curved atomizing medium supply hole is formed in the fuel supply member. In this case, there is a possibility that a swirling flow of liquid fuel into the injection nozzle is obstructed by the energy of a curved flow of the atomizing medium. In order to prevent the reduction of flow energy of liquid fuel, it is necessary to increase a pressure difference between the atomizing medium and the liquid fuel. As a result of the foregoing, there is a possibility of misfire, so that the turndown ratio can not be made sufficiently high.